How to share variable among more than 2 operation
Finally, I solved this problem using static variable.

class SelectFace(bpy.types.Operator):
    bl_label = "Select Face"
    selected_faces = None
    def __init__(self):
        SelectFace.selected_faces = None
    def execute(self, context):
        SelectFace.selected_faces = ... # get selected faces.
        .....
class OperationAboutSelectedFaces(bpy.types.Operator):
    bl_label = "Operation About selected Face"
    def __init__(self):
        selected_faces = SelectFace.selected_faces
    def execute(self, context):
        # some operation to "selected_faces"


Thanks for your advice.